Add "Add to .gitignore" action in project_panel

[korbindeman]

This PR adds a "Add to .gitignore" action to the project panel's right-click context menu. Similar to the "Restore File" action that I previously added, I frequently find myself wanting this in the project panel.

With the restore file option:

Notes:

• Implementation: The add_to_gitignore function is essentially copy-pasted from git_panel.rs.

• Error handling: Added toast notification on error, which is consistent with restore_file in project_panel and perform_checkout in git_panel.
  Note that add_to_gitignore in git_panel does NOT show a toast (just uses detach_and_log_err). I don't know if this is on purpose.
  To follow up, I can either: match the project_panel implementation to the git_panel one (no toast), or update the git_panel implementation to also show a toast on error.

• Menu grouping: Previously "Restore File" and "View File History" were in separate sections, but both relate to git. With this third git action, I grouped all three together under a single separator (see screenshot).
  We could also keep "View File History" separate and only group "Restore File" + "Add to .gitignore" together (both modify the working tree state in some way), if we don't want to alter the existing UI too much.

Release Notes:

• Added "Add to .gitignore" option to the project panel context menu for files in git repositories.

[cole-miller]

Thanks! Could we extract the code to add a path to .gitignore to a crate (maybe project) where both project_panel and git_ui can refer to it? Other than that this seems good.

[korbindeman]

Yes, done! I also made it work for directories as well (was just files).

[korbindeman]

Hey @cole-miller, just checking in on the changes I made addressing your feedback on Feb 2. Let me know if there's anything else needed!

[ChristopherBiscardi]

This is looking good. I've merged main and made some updates that were required from other refactors in the meantime.

This will create a new .gitignore if none exists, and add to if it does, which is super nice, thanks!